EP2451100A2 - Appareil et procédé pour synchroniser des dispositifs sans fil - Google Patents

Appareil et procédé pour synchroniser des dispositifs sans fil Download PDF

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Publication number
EP2451100A2
EP2451100A2 EP11250763A EP11250763A EP2451100A2 EP 2451100 A2 EP2451100 A2 EP 2451100A2 EP 11250763 A EP11250763 A EP 11250763A EP 11250763 A EP11250763 A EP 11250763A EP 2451100 A2 EP2451100 A2 EP 2451100A2
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Prior art keywords
series
access point
devices
central access
synchronization messages
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EP11250763A
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German (de)
English (en)
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EP2451100A3 (fr
Inventor
Michael A. Lynch
Radoslaw Romuald Zakrzewski
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Simmonds Precision Products Inc
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Simmonds Precision Products Inc
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Publication of EP2451100A2 publication Critical patent/EP2451100A2/fr
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/02—Details
    • H04J3/06—Synchronising arrangements
    • H04J3/0635—Clock or time synchronisation in a network
    • H04J3/0638—Clock or time synchronisation among nodes; Internode synchronisation
    • H04J3/0658—Clock or time synchronisation among packet nodes
    • H04J3/0661—Clock or time synchronisation among packet nodes using timestamps
    • H04J3/0667—Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L69/00—Network arrangements, protocols or services independent of the application payload and not provided for in the other groups of this subclass
    • H04L69/16—Implementation or adaptation of Internet protocol [IP], of transmission control protocol [TCP] or of user datagram protocol [UDP]
    • H04L69/165—Combined use of TCP and UDP protocols; selection criteria therefor
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W4/00—Services specially adapted for wireless communication networks; Facilities therefor
    • H04W4/70—Services for machine-to-machine communication [M2M] or machine type communication [MTC]

Definitions

  • the subject disclosure relates to communication in sensing, monitoring and control systems, and more particularly to synchronizing wireless devices used on board aircraft.
  • a mechanical diagnostic system may collect vibration data from multiple sensors. For correct interpretation of multi-dimensional vibration data, it is necessary to make sure that the acquired signals correspond to the same intervals in time.
  • the synchronization is typically achieved in one of two ways. The first approach is to devise a method to make sure that all sensors start their data acquisition at the same time. The second approach is to have all sensor nodes measure their local time according to well synchronized clocks.
  • the two approaches to synchronization are in fact equivalent. If it is possible to command several sensor nodes to perform a certain task starting at the same time, then the task may involve starting or resetting their clocks. If clock resets are done simultaneously, then the clocks will be closely synchronized for some time provided their rates do not differ too much. On the other hand, if the clocks are tightly synchronized, then the sensors may be commanded to perform the task of interest, such as to start data acquisition, at particular time values according to their clocks. In other words, the respective sensor's reading can be correlated in such way that the readings correspond to the same physical time instance.
  • the Karl/Willig approach rests on the assumption that propagation and processing delays of the messages are symmetric and statistically constant in time. That is, it is assumed that any random delays in delivering a message from wireless node N1 to the wireless node N2 are distributed in the same way as message delivery delays from wireless node N2 to wireless node N1, and this statistical distribution is stationary in time. Furthermore, the accuracy of the resulting clock synchronization depends on the variability of message delivery delays. Generally, as the variance of message delivery delays gets larger, so does the synchronization error. Therefore, for synchronization to be accurate in the Karl/Willig approach, it is desirable to have as little variance in message delivery delays as possible.
  • COTS commercial off-the-shelf
  • TCP Transmission Control Protocol
  • UDP User Datagram Protocol
  • TCP may be more attractive than UDP, because TCP reduces or eliminates the need to add customized reliability mechanisms.
  • the subject disclosure has recognized and addressed the need to effectively synchronize sensors and other system elements that communicate wirelessly using high level commercial protocols.
  • commercially available high level protocols such as TCP
  • TCP are used for reliable wireless transmission of sensor data.
  • the method switches operation to a less complicated, potentially less reliable, but simpler protocol with limited and/or more consistent latency, such as UDP.
  • UDP more consistent latency
  • the subject technology is directed to a method for synchronizing a plurality of wireless devices including the steps of sending a set of multiple reference messages from an access point to each of the plurality of devices, recording, at each device, a timestamp corresponding to each of the reference messages that is received, sending from each of the wireless devices a response to a request for the timestamps corresponding to the reference messages, and determining relative clock offsets between each of the wireless devices using the timestamps.
  • the devices may be sensors, control systems, monitoring systems and the like.
  • the subject technology is a method for synchronizing devices from a central access point node, the method including the steps of utilizing a low latency protocol between the central access point node and the devices to transmit a series of reference broadcasts, calculating and applying relative clock offsets for each device based on the series of reference broadcasts, and transmitting and receiving data acquisition commands and responses between the central access point node and the devices using a high reliability protocol different from the low latency protocol.
  • the devices may be wireless sensors.
  • the low latency protocol may be UDP and the high reliabiliability protocol may be TCP.
  • the series of reference broadcasts can include timestamp pairs sent to first and second devices.
  • the method may also include accounting for failure in delivery of a portion of the series of reference broadcasts by utilizing each broadcast received at the first and second device nodes, and calculating at the central access point node an intermediate quantity for each device based on timestamps sent from each device.
  • the method may calculate the clock offset using a subset of the series of reference broadcasts.
  • the subset can be broadcasts with minimal propagation and processing delays.
  • the subset could be a simple percentage such as 50% of the series of reference broadcasts, only messages with delays below a predetermined threshold, a certain amount such as a statistically significant sample size, and the like.
  • the method also can calculate a clock drift for at least one of the devices based on the series of reference broadcasts.
  • Another embodiment is a method for synchronizing devices from a central access point node including the steps of transmitting a series of reference broadcasts between the central access point node and the devices, accounting for failure in delivery of a portion of the series of reference broadcasts, collecting timestamp data based on the series of reference broadcasts, and calculating and applying relative clock offsets for each device based on the timestamp data of the series of reference broadcasts.
  • the devices can be a first and second device in which the timestamp data from the first device is decoupled from the timestamp data of the second device.
  • the subject technology is also directed to an apparatus for synchronizing devices including a central access point node configured to utilize a low latency protocol to transmit a series of reference broadcasts to the devices, transmit and receive data acquisition commands and responses from the devices using a high reliability protocol different from the low latency protocol, and calculate and apply relative clock offsets for each device based on the series of reference broadcasts.
  • the apparatus may also account for failure in delivery of a portion of the series of reference broadcasts by recording timestamp data at the devices to send to the central access point prior to the central access point calculating the relative clock offsets.
  • Figure 1 is a flowchart illustrating a synchronization method for wireless sensors in accordance with the subject technology.
  • Figure 2 is an exemplary series of reference broadcasts between the central access point AP and wireless sensor nodes N1, N2 in accordance with the subject technology.
  • Figure 3 is a flowchart illustrating another synchronization method for wireless sensors in accordance with the subject technology.
  • Figure 4 is an exemplary series of synchronization messages between the central access point AP and wireless sensor node N1 in accordance with the method of Figure 3 .
  • Figure 5 is another exemplary series of synchronization messages between the central access point AP and wireless sensor node N1 in accordance with the method of Figure 3 .
  • FIG. 1 a flowchart 100 illustrating a synchronization method for wireless sensors in accordance with the subject technology is shown.
  • the synchronization method 100 is for a system having sensors that use the highly reliable TCP protocol for most message exchanges including data acquisition commands from a central access point node AP and transmission of the acquired sensor data to the central access point node AP.
  • the method 100 switches to a simpler albeit less reliable protocol such as UDP. Even though not all UDP messages may reach their destination, the messages that are completed have lower and less random delivery delays than the more reliable TCP.
  • the synchronization method of Figure 1 may utilize, in whole or in part, relative synchronization of sensors via reference broadcast as disclosed in U.S. Patent Application No. 12/799,087, filed on April 16, 2010 and entitled SYNCHRONIZING WIRELESS DEVICES, which is incorporated by reference herein (hereinafter the '087 application).
  • the method 100 illustrates message exchange sequences between the central access point node AP, a first wireless sensor node N1, and a second wireless sensor node N2.
  • step S1 an initial series of reference broadcast messages are sent by UDP from the central access point node AP to the wireless sensor nodes N1, N2.
  • the arrival times of the reference broadcast messages at the sensor nodes N1, N2 are used to calculate relative offsets between the clocks of nodes N1, N2.
  • the method 100 is equally applicable to a plurality of any number of sensors.
  • UDP By using UDP for this initial series of broadcasts, it is possible to reduce the random spread of message delays and thus to improve synchronization accuracy. Because UDP does not include any built-in reliability mechanisms, some of the reference broadcast messages may be lost. Some complications may arise when different messages are lost between the central access point node AP and sensor node N1 and between the central access point node AP and sensor node N2. As a consequence, the timestamp information from nodes N1, N2 may correspond to different sets of reference broadcasts messages. The method 100 resolves the difficulty associated with lost messages as discussed hereinbelow.
  • Figure 1 does not depict all messages exchanged between the nodes AP, N1, N2.
  • requests for timestamp data or for the acquired sensor data may be sent from the central access point node AP to the sensor nodes N1, N2.
  • the sensor nodes N1, N2 may be sending the data to the central access point node AP based on a schedule, which may be predetermined beforehand, included as additional payload in reference broadcasts, or in data acquisition commands.
  • the timestamp data from the sensor nodes N1, N2 may be bundled together with the acquired sensor data after acquisition is completed.
  • the central access point node AP may perform post-hoc calculation of the relative clock offsets.
  • step S1 utilizes a low-latency, relatively low-reliability protocol for the initial reference series of timestamp messages. Since a larger number of the initial messages may not be delivered, the decrease in accuracy of calculating the relative clock offset between the sensor nodes N1, N2 must be overcome. For example, if the central access point node AP sends a number M of reference broadcast messages, let t 1 (i) and t 2 (i) be the receive timestamps at sensor nodes N1, N2, according to the corresponding local clocks.
  • the number of individual offset estimates O (i) 12 may become small. For example, if 10% of messages fail to arrive at the sensor nodes N 1, N2, then the number of available estimates in the set O (i) 12 is reduced by up to 20%. In a more severe case, a loss of 40% of messages at each sensor node N1, N2 may result in up to 80% of clock offset estimates in set O (i) 12 being lost, or equivalently only 20% of those estimates are available. In such a case, in order to achieve the desired accuracy of clock offset estimation, it might be necessary to increase the number of reference broadcasts by five times. Further, from the point of view of energy consumption, it is desirable to minimize the number of reference broadcasts sent by the central access point node AP.
  • the sensor nodes N1, N2 send timestamps to the central access point node AP.
  • O i 20 t 2 i - t 0 i
  • O i 10 t 1 i - t 0 i
  • the quantities O (i) 20 and O (i) 10 could be treated as instantaneous clock offsets between the central access point node AP and the sensor nodes N1, N2. Because of the nonzero propagation and processing delays, the numbers O (i) 20 and O (i) 10 do not allow any accurate synchronization between the central point access node AP and sensor node N1, or between the central point access node AP and sensor node N2.
  • O mean 20 mean O i 20
  • O mean 10 mean O i 10
  • O mean 12 O mean 10
  • O mean 20 mean O i 12
  • the intermediate calculation of the means O (i) 20 and O (i) 10 in the method 100 may not be necessary when every message is delivered.
  • use of the intermediate quantities O (i) 20 and O (i) 10 is advantageous when some of the reference broadcast messages of step S1 are not received by the sensor nodes N 1, N2.
  • the central access point node AP calculates the relative clock offsets between the sensor nodes N1, N2 using the intermediate quantities O (i) 20 and O (i) 10 . For example, if 25% of the broadcast messages are lost by sensor nodes N1, N2, up to 50% of timestamp pairs t 1 (i) , t2 (i) may be not not available for synchronization purposes. Hence, to accomplish the same desired synchronization accuracy, the number of reference broadcast needs to be doubled.
  • the number of timestamp pairs t 1 (i) , to (i) is 75% of the non-loss case (e.g., only a 25% shortfall).
  • the intermediate estimate O mean 20 may be calculated using 75% of all timestamp pairs t 2 (i) , to (i) .
  • the overall clock offset estimate O mean 12 O mean 10 - O mean 20 calculated from the intermediate quantities O (i) 20 and O (i) 10 is more accurate. If desired, to achieve the same synchronization accuracy, the number of reference broadcast needs to only be increased by one third.
  • the method 100 proceeds.
  • the method 100 is ready for data acquisition, so the central access point node AP commands the sensor nodes to acquire data using TCP.
  • the sensor nodes N1, N2 collect the relevant data.
  • the sensor nodes N1, N2 send the acquired data to the central access point node AP using TCP as shown in step S6.
  • an exemplary series of reference broadcasts between the central access point AP and wireless sensor nodes N1, N2 is referred to generally by the reference numeral 200 and illustrates a possible examplary data with which the method 100 of Figure 1 may be utilized.
  • the reference message j is not received by the sensor node N2, but the reference message j is received by the sensor node N1. Consequently, the timestamp pair t 1 (j) , t 0 (j) may still contribute to the intermediate calculation of O mean 10 even though there is no timestamp t 2 (j) to contribute to the intermediate calculation of O mean 20 .
  • the timestamp pair t 2 (k) , t0 (k) is used in calculation of O mean 20 even though the missing timestamp t 1 (k) cannot be used in calculation of O mean 10 .
  • O mean 10 may be calculated from N-K 1 timestamp pairs (t 1 (i) t 0 (i)
  • O mean 20 may be calculated from N-K 2 timestamp pairs (t 2 (i) , t 0 (i) ).
  • N-K 1 -K 2 timestamp pairs (t 2 (i) , t 1 (i) ) are used to calculate O mean 12 .
  • the method 100 performs a clock offset calculation based on the broadcast messages for which the propagation and processing delay is the shortest. Typically, the shorter the delay, the more accurate the resulting calculation.
  • O mean 10 min O i 10 where the minima are taken over the reference broadcasts received by the sensor nodes N2, N1, respectively.
  • a median approach may be used to estimate O 10 and O 20 to minimize the influence of outliers.
  • Another possibility may be to use a trimmed mean approach, or a trimmed median approach, when a fixed number of largest values of O (i) 10 and O (i) 20 would be excluded from calculations.
  • the estimation of the clock offset O 10 between the central access point node AP and the sensor node N1 is decoupled from the estimation of the clock offset O 20 between the central access point node AP and the sensor node N2, and then the resulting estimates are combined into the final clock offset O 12 between the sensor nodes N1, N2.
  • timestamp information associated with a particular reference broadcast is utilized as long as at least one sensor received that message.
  • FIG. 3 a flowchart 300 illustrating another synchronization method for wireless sensors in accordance with the subject technology is shown.
  • the method 300 utilizes similar principles and hardware structure to the method 100 described above. Accordingly, like reference numerals and letters are used to indicate like elements.
  • the primary difference of the method 300 in comparison to the method 100 is the pair-wise synchronization between the central point access node AP and the sensor nodes N1, N2 based on quadruple timestamp exchanges.
  • the method 300 decouples the calculations involving messages from the central access point node AP to the sensor nodes N1, N2 as well as messages from the sensor nodes N1, N2 to the central access point node AP. By doing so, both the accuracy and the duration of the method 300 are improved.
  • the central access point node AP and the sensor nodes N1, N2 exchange timestamps using the simple UDP protocol to reduce latency. It is noted that the timestamp exchanges may occur at different times as shown in Figure 3 . As mentioned above, a potential drawback of using the UDP protocol is that some of the timestamp messages may be lost. On the other hand, the timestamp messages that do arrive may be delivered faster and with more certainty, due to fewer software layers involved in handling the messages, which allows for more accurate synchronization. The rest of the communication, including the data acquisition commands from the central access point node AP to the sensor nodes N1, N2, may be performed via the more complex TCP protocol.
  • the central access point node AP calculates the relative clock offset for the sensor node N1 based on the timestamp exchange.
  • the central access point node AP calculates the relative clock offset for the sensor node N2. Once the desired overall clock offsets are determined, the method 300 proceeds.
  • the method 300 is ready for data acquisition, so the central access point node AP commands the sensor nodes N1, N2 to acquire data using TCP.
  • the sensor nodes N1, N2 collect data.
  • the sensor nodes N1, N2 send the acquired data to the central access point node AP using TCP as shown in steps S15, S16, respectively.
  • the depiction of method 300 in Figure 3 is only meant to illustrate graphically one possible rendering of the subject technology, and does not include some of the messages that need to be exchanged for clarity.
  • the transfer of sensor data to the central point access node AP may involve data requests or acknowledgements from the central point access node AP to the sensor nodes N1, N2.
  • the timing and sequence of events may differ from that shown in the Figure 3 .
  • the clock offsets between the central access point node AP and both sensor nodes N1, N2 may be calculated only after all timestamp exchanges are finished.
  • the two series of timestamp exchanges of step S10 between the central point access node AP and the sensor node N1 and between the central point access node AP and the sensor node N2 may be interleaved instead of occurring one after the other.
  • an exemplary series of synchronization messages 400 between the central access point AP and wireless sensor node N1 is referred to generally by the reference numeral 400 and illustrates a possible example of a modification of the clock offset calculations in the quadruple timestamp approach of method 300.
  • losses of timestamp messages are comparatively more likely than in the method 100 of Figure 1 because the losses may occur on the way from the central access point node AP to the sensors N1, N2 as well as on the return route. For simplicity, only messages between the central point access node AP and the sensor node N1 are shown.
  • the timestamps sent from the central access point node AP are denoted as t 1 (i)
  • the timestamps received at the sensor node N1 are denoted as t 4 (i)
  • the timestamps sent from the sensor node N1 are denoted as t 5 (i)
  • the timestamps received at the central point access node AP are denoted as t 8 (i) .
  • a message carrying timestamp t 1 (2) sent from the central access point node AP to the sensor node N1 was lost.
  • a message carrying timestamps t 1 (3) , t 4 (2) , t 5 (2) sent from the sensor node N1 to the central access point node AP was also lost.
  • O 10 (i) is then used to calculate the overall clock offset between the central access point node AP and the sensor node N1.
  • Several methods are known for subsequently determining the clock offset based on this data and, thus, any estimation techniques now known or later developed may be utilized.
  • the central access point node AP initiates four timestamp exchanges using messages carrying timestamps t 1 (1-4) but only two time stamp responses carrying timestamps t 5 (1) , t 5 (4) are returned by the sensor node N1 (the second response t 5 (3) fails).
  • a drawback of simply increasing the number of timestamp messages sent is the increased time of the overall synchronization operation. For example, after a message from the central access point node AP to the sensor node N1 is lost, the central access point node AP waits for a response from the sensor node N1. Thus, the next message to the sensor node N1 is sent only after the central access point node AP times out when it becomes apparent that no response will be coming. This unnecessary waiting time translates into unnecessary energy expenditure while both nodes AP, N1 are in an active state.
  • the method 300 overcomes the lost messages problem by observing that in the formula to calculate O 10 (i) , the two timestamp differences t 8 (i) - t 5 (i) and t 4 (i) - t 1 (i) are independent, because two timestamp differences t 8 (i) - t 5 (i) and t 4 (i) - t 1 (i) correspond to two distinct messages: the difference t 4 (i) - t 1 (i) corresponds to the message from the central access point AP to the sensor node N1, and the difference t 8 (i) -t 5 (i) corresponds to the message from the sensor node N1 to the central access point AP. Because the associated propagation and processing delays do not depend on each other, consecutive messages exchanged between the two wireless devices are not required.
  • FIG. 5 another exemplary series of synchronization messages 500 between the central access point AP and wireless sensor node N1 in accordance with the method of Figure 3 is shown.
  • the scheme utilized in Figure 5 takes advantage of the sensor node N1 not needing to respond to each and every message carrying timestamps t 1 (1-4) sent by the central access point AP. Instead, the sensor node N1 collects the timestamp data t 1 (1,3,4) and t 4 (1,3,4) associated with a sequence of received messages from the central access point AP, and then responds with a sequence of response messages carrying timestamps t 5 (1-4) .
  • O mean f mean O i f
  • O mean r mean O i r where the averages are taken over the respective collections of the available timestamp differences.
  • the modified overall clock offset estimate formula becomes clearly advantageous when there may be message losses in both directions.
  • the data is used more efficiently, and fewer messages need to be sent.
  • the waiting period following message losses is greatly reduced. Because either of the wireless nodes sends a sequence of messages without waiting for responses, the central access point node AP avoids unnecessary waiting, which may allow a significant reduction in energy use.
  • various estimation methods may be used for clock offset calculation. This may involve median, trimmed median, or other estimators now known or later developed.
  • the set of timestamp pairs t1 (i) , t 4 (i) corresponding to the first sequence of messages may be transmitted from the sensor node N1 to the central access point node AP at the end of the whole process, in a separate message or messages using the high-reliability TCP protocol.
  • the exact time instant when this information reaches the central access point node AP is not critical, as long is the transmission occurs soon after the last timestamp message from sensor node N1 reaches the central access point node AP.
  • the set of timestamp pairs t1 (i) , t 4 (i) may be sent from the sensor node N1 to the central access point node AP between two sequences of messages.
  • the timestamp data may be included in the timestamp messages from sensor node N1 to the central access point node AP, possibly repeated in consecutive messages to account for the possible message losses.
  • the timestamps t1 (i) , t 4 (i) are recorded by the sensor node N 1, the information only needs to be sent to the central access point node AP in time for the central access point node AP to calculate the clock offset.
  • the clock drift may be disregarded. If this is not the case, the subject technology may be extended to include both clock offset and clock drift rate estimation.
  • the Karl/Willig approach proposes a drift estimation using multiple data points and assuming that sensor node drift is constant through an interval of interest. It is also envisioned that the subject technology may apply to a subset of nodes. In such an application, the central access point AP can multicast or send out synchronizing messages to selected sensors and/or nodes instead of using broadcast messages directed to all nodes within the network.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Computer Security & Cryptography (AREA)
  • Mobile Radio Communication Systems (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)
EP11250763.7A 2010-11-08 2011-09-09 Appareil et procédé pour synchroniser des dispositifs sans fil Pending EP2451100A3 (fr)

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US12/941,226 US8489776B2 (en) 2010-11-08 2010-11-08 Apparatus and method for synchronizing wireless devices

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Publication number Priority date Publication date Assignee Title
EP2675224A1 (fr) * 2012-06-13 2013-12-18 Simmonds Precision Products, Inc. Système et procédé de synchronisation de dispositifs sans fil sans envoyer des données d'horodateur
EP2926604A4 (fr) * 2012-11-30 2015-12-09 Valmet Automation Oy Procédé et système de mesure d'un capteur multicanal
US10021189B2 (en) 2012-11-30 2018-07-10 Valmet Automation Oy Multi-channel sensor measurement method and system

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US8489776B2 (en) 2013-07-16
US20120117272A1 (en) 2012-05-10

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